The present invention is directed to a method of inserting steel tendons, particularly strands or bundles of strands of steel wire, into ducts formed in a concrete structural member, such as by encasing sheathing tubes in the concrete
In erecting prestressed concrete structures, tendons are inserted either within the sheathing tubes located in the form work before the concrete is poured or are subsequently introduced into the ducts formed in the concrete structural member by encasing the sheathing tubes in the concrete. This second procedure has the special advantage in a heavy prestressing member intended for high tensioning forces, that in the preparation of form work and the installation of slack nonprestressed reinforcement of the structural member, it is necessary only to build in light sheathing tubes. Subsequently, the considerable weight of the tendons is applied and can be carried by the concrete structural member in the hardened condition.
Tendons can be placed in the prepared ducts either using a pulling member introduced before hand, that is, a so-called pilot wire or the tendons can be pushed in by appropriate pushing devices. Motor driven pushing devices including motor driven friction rollers engaging one tendon provide the pushing forces acting in the axial direction of the tendon and such devices are known.
During the pulling or pushing method of inserting the tendons into the ducts, the friction of the tendons developed at the surface of the sheathing ducts must be overcome. Overcoming the friction is relatively easy in short, generally straight tendons. Problems arise, however, when the tendon is directed around a large redirecting angle or around a tight small radius curve. It is especially difficult if tendons must be directed around a complete circle, such as in a spherical or cylindrical container.
Such a problem occurs in a pressure vessel for storing, producing or transporting compressed gases where the prestressing members are in the form of closed rings tensioned as a result of the pressure of the medium within the container causing a radial widening of parts of the container separated from one another by expansion joints.
When tendons are pulled into the sheathing ducts, the force available is reduced due to friction losses to about 30 per cent of the initial force when the tendons are moved through reversing or redirecting angles of 360.degree. with a useful friction coefficient of 0.2. Installation of prestressing members around reversing angles larger than 360.degree. does not appear to be possible, because of the snubbing effect developed during the pulling operation with angles of greater than 360.degree.. When tendons are pushed into the ducts, frictional forces develop due to the continuous buckling load on the tendon and because of contact at the surface of the sheathing tubes, accordingly, pushing tendons around such redirecting angles appears to be inconceivable.